Highly alloyed stainless steels and nickel based alloys are now utilized in environments which produce significant localized corrosion in many other metals and alloys. The excellent pitting corrosion resistance of these highly alloyed stainless steels and nickel based alloys is due to the high alloy composition, which is believed to inhibit the anodic corrosion processes. Of the beneficial alloying elements in stainless steels, chromium is the most important because it forms a bipolar passive film, see A. R. Brooks, C. R. Clayton, K. Doss and Y. C. Lu, J. Electrochem. Soc., Vol. 133, 2459, (1986). To date, the alloy development approach has been to increase the amount of alloyed chromium, molybdenum and nitrogen in order to improve pitting corrosion resistance. However, crevice corrosion remains a problem in these alloys. For example, it can be manifest as under-deposit corrosion, as has been found in recent ocean tests even in steels with high molybdenum and chromium contents, see M. B. Ives, in Proceedings "Applications of Stainless Steels '92", Jemkontoret, Stockholm, 436 (1992).
The major difference between crevice and pitting corrosion involves the initiation stages. Crevice corrosion in aerated solutions involves an oxygen concentration cell. Furthermore, in the later stages of localized corrosion development, cathodic reduction of the depolarizers on the large areas surrounding the attacked site is necessary to support the high rate of anodic dissolution. It has been disclosed by Y. C. Lu, J. L. Luo and M. B. Ives, ISIJ International, Vol. 31, 210 (1991), that the enhanced cathodic reduction of oxidant adjacent to a localized attack site produces an increase of localized corrosion. Thus a powerful means of preventing crevice corrosion would be to constrain or significantly inhibit the cathodic reactions such as oxygen reduction, hydrogen evolution and the like.
It has been previously reported that cerium ion-implantation in UNS S31603 stainless steels effectively inhibits the reduction of oxygen and protons, reducing the rate by more than two orders of magnitude, see Y. C. Lu and M. B. Ives, Corrosion Sci., Vol. 34, 11, 1773 (1993). Also, the anodic (passive) current density is reduced by more than one order of magnitude for UNS S31603 stainless steel after cerium implantation. Consequently, cerium ion implantation improves the crevice corrosion resistance of UNS S31603 stainless steel as determined by both anodic polarization in aerated 0.1 M Na.sub.2 SO.sub.4 +0.6 M NaCl solution and by the ASTM G48 B crevice test in 10% ferric chloride hexahydrate solution. However, ion-implantation is not readily amenable to economically treating large surface areas materials. Further, ion-implantation may induce radiation damage at the surface of the metal or alloy which may have detrimental structural effects so that ion-implantation has practical limitations.
The nickel based alloys and high alloy stainless alloys are most frequently used in specific aggressive aqueous corrosion environments. These alloys can benefit considerably from enhanced corrosion resistance by controlling the cathodic reaction rates. However, more commonly used alloys in industrial applications such as 18-8 stainless, UNS S30400 or the Mo containing alloy, UNS S31603, can also benefit from the effects of reduced cathodic reaction rates. The ferritic stainless steels, which are the least corrosion resistant of the stainless family, would advantageously benefit from increased corrosion resistance by any mechanism. A common component in these alloys is the presence of a passive layer.
Aluminum and aluminum alloys, although extremely different in structure than the ferrous and nickel alloys, also possess passive layers and would benefit from increase corrosion inhibition. Corrosion and corrosion induced failure is a major problem associated with aluminum alloys. Aluminum alloys are widely used in corrosive environments, for example in automotive applications such as brazed aluminum heat exchangers, coolers, evaporators, radiators and the like. Known methods of corrosion protection of aluminum and aluminum alloys involve the use of chromate ions to form conversion coatings on the alloys. Environmental concerns associated with chromate ions are a drawback to widespread use of this technique. Other strategies for increasing corrosion resistance of aluminum based alloys based on physical deposition methods such as sputtering are inherently limited since the area being coated is by line-of-sight from the source.
Cerium containing solution treatment has been effective in improving the localized corrosion resistance of aluminum alloys. For example, it has previously been reported that chemical passivation of aluminum alloys immersed in cerium chloride solutions for 7 days or longer produces a conversion coating on the aluminum alloy exhibiting increased corrosion resistance, see F. Mansfield, S. Lin, S. Kim and H. Shih, J. Electrochem. Soc., Vol. 137, 78 (1990). In order to speed up the production of the conversion coating, the aluminum alloys have been dipped into hot cerium salt solutions followed by direct current (DC) anodic polarization in a molybdate solution to produce an anodized passive layer containing Ce and Mo as disclosed in F. Mansfield, V. Wang and H. Shih, J. Electrochem. Soc., Vol. 138, L74 (1991). Alternating current (AC) passivation of aluminum alloys in the same types of cerium salt solutions has also been used to form anodized layers exhibiting corrosion resistance as disclosed in H. Shih, V. Wang and F. Mansfield, Corrosion 91, Paper #136, NACE, Houston (1991). The use of rare earth metal chlorides as inhibitors for aluminum alloys in NaCl has been disclosed in D. R. Arnott, B. R. W. Hinton and N. E. Ryan, Corrosion, Vol. 45, 12 (1989).
U.S. Pat. No. 5,194,138 issued to Mansfeld is directed to a multi-step process for forming a corrosion resistant aluminum surface coating by exposure first to a cerium non-halide solution followed by exposure to an aqueous cerium halide (chloride) solution. The purpose of this multi-step treatment process is to grow or continue to grow, in successive steps, a uniform, non-porous thick protective oxide coating to protect the Al surface against anodic attack causing pitting corrosion. This patent also teaches exposing the aluminum surface to molybdenum solutions and electrochemically positively charging the surfaces into the passive region to provide an anodically grown oxide coating. Regardless, the essence of this process is to produce an improved barrier oxide layer by precipitation of Ce (or Ce and Mo) in the growing oxide film to reduce porosity and increase electrical resistivity in the chemically or electrochemically formed films. A drawback to this method is the length of time required to grow a sufficiently thick oxide coating, i.e on the order of hours, and the fact that the efficacy of the thick protective coating depends in part on its uniformity. Achievement of the necessary uniformity presents practical limitations in terms of process treatment rate, or process controls.
U.S. Pat. No. 5,221,371 issued to Miller discloses non-toxic corrosion resistance conversion coatings for aluminum and Al alloys. The process is a multi step process using acidic solutions comprising cerium chloride and potassium permanganate alone or in combination with strontium chloride. U.S. Pat. No. 5,356,492 issued to Miller is very similar to '371 but substitutes hydrogen peroxide for potassium permanganate.
Patent publication WO-A-95/08008 is directed to a cleaning solution for use in a multi-step method for chemically cleaning surfaces of aluminum and its alloys. The method provides a means of pre-treating Al alloy surfaces prior to application of other coatings such as paint layers and the like.
U.S. Pat. No. 5,362,335 issued to Rungta discloses a four step process directed to forming a corrosion resistant surface on aluminum alloys only using cerous chlorides solutions. A bohmite film is first formed on the aluminum alloy surface after which the bohmite coated sample is then subjected to a drying step at about 200.degree. F.
For the foregoing reasons, there has been a need for a simple, inexpensive, and rapid surface treatment for increasing the corrosion resistance of industrially important metals and alloys such as copper and copper alloys, chromium, molybdenum, ferritic and austenitic stainless steels, nickel based alloys, aluminum alloys and the like which is rapid and environmentally safe.